It has been proposed to use force limiters, and in particular energy-absorbing force limiters, in connection with safety devices provided in motor vehicles such as, for example, seat-belts. The function of a force limiter is to yield, whilst absorbing energy, if the force limiter is subjected to a force in excess of a predetermined threshold. Whilst it is always desirable to ensure that an occupant of a vehicle should not hit any part of the vehicle with a substantial relative speed, it is known that it is undesirable to apply excessive forces to the torso of an occupant through a seat-belt in order to achieve this objective, since the application of high forces may injure the occupant of the vehicle. Thus, the effect of a force limiter is such that when a very substantial acceleration is to be imparted to a vehicle occupant by a seat-belt, the seat-belt will yield slightly, whilst the force limiter absorbs energy, thus minimising the risk of injury arising.
Various types of force-limiting energy-absorber have been proposed previously, including energy-absorbers which rely on the deformation of a metal element. Such arrangements are relatively simple and have a very predictable operating characteristic, but suffer from the disadvantage that it is not easy to vary the level of energy absorbed in response, for example, to the weight of the occupant of the seat or the position of the occupant of the seat immediately before commencement of the accident.
It has also been proposed to provide an adjustable force limiter which incorporates a first component and a second component that is moveable relative to the first component, the force limiter including an arrangement that provides an energy-absorbing movement-resisting effect to resist movement of the second component relative to the first component. GB-A-2,386,350 discloses a force limiter of this type in which friction is used to provide the movement-resisting effect and to absorb energy. In one embodiment, a retractor reel spool moves relative to a housing and a frictional effect can be applied to parts of the spool by expanding a series of piezo-electric plates which form a stack of plates. A control signal is supplied to expand the piezo-electric plates, and thus the frictional force, and the movement-resisting effect, can be controlled. However, the frictional force is not solely determined by the degree of expansion of the piezo-electric plates, but is also dependent upon the temperature of the components which are frictionally engaged, as this can vary the absolute value of the coefficient of friction. In the described arrangement, therefore, a control loop is provided in which a desired value of “belt force” is compared, in a comparator, with a value representing the reel “belt force” which has to be measured. Such a control loop is relatively “slow” and, of course, in a real accident situation, the requirement to absorb energy, in a precisely appropriate manner, can arise very soon after the commencement of the accident, and the duration of the period in which the energy is to be absorbed can, itself, be very short. Thus a “slow” control loop is very undesirable.
The present invention seeks to provide an improved energy absorbing force limiter.